<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style type="text/css">
        pre {
            font-family: monospace;
            display: block;
            white-space: pre;
            font-size: 2em;
        }
    </style>
</head>
<body>
    <script type="text/javascript">

        const MIN = 0;
        const MAX = 0xFFFFFFFF;

        function zeropad(a, n) {
            return "0".repeat(n - a.length) + a;
        }

        // const encodeNumber = n => zeropad((n >>> 0).toString(16), 8);

        function encodeNumber(n) {
            const a = (n & 0xFFFF0000) >>> 16;
            const b = (n & 0xFFFF) >>> 0;
            //return zeropad(a.toString(16), 4) + zeropad(b.toString(16), 4);
            return String.fromCharCode(a, b);
        }

        function decodeNumber(s) {
            const a = s.charCodeAt(0);
            const b = s.charCodeAt(1);
            //return `${a.toString(16)} ${b}`;
            return ((a << 16) | b) >>> 0;
        }

        function formatArg(a) {
            if (typeof a === "string") {
                return `"${a}"`;
            }
            if (Array.isArray(a)) {
                return `[${a.map(formatArg)}]`;
            }
            return a+"";
        }
        function cmp(a, b) {
            let value;
            try {
                const result = indexedDB.cmp(encodeNumber(a), encodeNumber(b));
                if (result < 0) {
                    value = "a < b";
                } else if (result === 0) {
                    value = "a = b";
                } else if (result > 0) {
                    value = "a > b";
                }
            } catch(err) {
                value = err.message;
            }
            return `cmp(${formatArg(a)} as ${formatArg(encodeNumber(a))},\n    ${formatArg(b)} as ${formatArg(encodeNumber(b))}): ${value}`;
        }

        try {
            const tests = [
                // see https://stackoverflow.com/questions/28413947/space-efficient-way-to-encode-numbers-as-sortable-strings
                // need to encode numbers with base 256 and zero padded at start
                // should still fit in 8 bytes then?
                (cmp) => cmp(9, 100000),
                (cmp) => cmp(1, 2),
                (cmp) => cmp(MIN, 1),
                (cmp) => cmp(MIN, MIN),
                (cmp) => cmp(MIN, MAX),
                (cmp) => cmp(MAX >>> 1, MAX),
                (cmp) => cmp(0x7fffffff, 0xffff7fff),
                (cmp) => cmp(MAX, MAX),
                (cmp) => cmp(MAX - 1, MAX),
            ];

            for (const fn of tests) {
                const txt = document.createTextNode(fn(cmp));
                const p = document.createElement("pre");
                p.appendChild(txt);
                document.body.appendChild(p);
            }
        } catch(err) {
            alert(err.message);
        }

        let prev;
        for (let i = MIN; i <= MAX; i += 100) {
            if (decodeNumber(encodeNumber(i)) !== i) {
                document.write(`<p>${i} decodes back to ${decodeNumber(encodeNumber(i))}</p>`);
                break;
            }
            if (typeof prev === "number") {
                if (indexedDB.cmp(encodeNumber(prev), encodeNumber(i)) >= 0) {
                    document.write(`<p>${i} <= ${prev}</p>`);
                    break;
                }
            }
            prev = i;
        }
        document.write(`<p>check from ${MIN} to ${prev}</p>`);
    </script>
</body>
</html>
 
